Issue #789290: make sure that hash(2**63) == hash(2.**63) on 64-bit

Issue #789290: make sure that hash(2**63) == hash(2.**63) on 64-bit
platforms.  The previous code was fragile, depending on the twin
accidents that:

  (1) in C, casting the double value 2.**63 to long returns the integer
      value -2**63, and
  (2) in Python, hash(-2**63) == hash(2**63).

There's already a test for this in test_hash.